As disclosed in Non-Patent Document 1 listed in [0010], a step-flow growth is known in one mode of crystal growth. In the present specification, crystal growth in the step-flow mode is called step-flow growth. As shown schematically in FIG. 8, a repeating shape of a step 28 and a terrace 26 is observed on a surface of a single crystal in step-flow growth. When crystal growth conditions are arranged, crystal growth proceeds at the positions of the steps 28, and the positions of the steps 28 proceed as shown by the arrow 30. When the step 28 reaches an end 32, a single crystal layer 24 has become thicker by the height of the step 28. Moreover, the height of the steps 28 and the inclination angle of the terraces 26 of FIG. 8 are shown in an exaggerated manner; the actual height of the steps 28 is between one and several atomic layers, and the inclination angle of the terraces 26 is extremely small.
In addition to step-flow growth, the two-dimensional nucleation and growth is also known. FIG. 9 schematically shows the surface of a single crystal layer during two-dimensional nucleation and growth. In two-dimensional nucleation and growth, crystal growth starts at a random location of a flat surface. Since the start position of crystal growth is determined probabilistically, on a surface 18 of a single crystal layer grown by two-dimensional nucleation and growth, planes 18a, 18b, 18c, etc. having different heights coexist, and a level difference 20 travels irregularly.
It is known that a surface of a single crystal layer grown by step-flow growth is flatter than a surface of a single crystal layer grown by two-dimensional nucleation and growth. Further, step-flow growth is easier to control than two-dimensional nucleation and growth. For example, in case of growing a single crystal doped with impurities, the concentration of impurities, and the position of impurities in the crystal structure, etc. can be managed more strictly by step-flow growth than by two-dimensional nucleation and growth. In the present specification, obtaining a result of crystal growth by arranging the conditions for crystal growth is called promoting crystal growth.
It is reported in Non-Patent Document 2 listed in [0010] that a crystal surface which is flat and has a steep heterointerface can be formed by the step-flow growth of a group-III nitride single crystal. Since a semiconductor device made of a group-III nitride single crystal uses a surface or heterointerface for a channel or drift region, a flat surface, flat interface, or a crystal structure with little disturbance works favorably. Non-Patent Document 2 reports that characteristics of the semiconductor device are improved by using a group-III nitride single crystal grown by step-flow growth.
It is reported in Non-Patent Document 3 listed in [0010] that the formation of deep levels can be suppressed and current collapse can be reduced by promoting the step-flow growth of a group-III nitride single crystal.
Techniques for promoting step-flow growth of a group-III nitride single crystal are taught in Non-Patent Documents 2, 4, 5 listed in [0010]. As shown schematically in FIG. 8, in these techniques, crystal growth of the group-III nitride single crystal 24 is performed on a substrate 22, this being a sapphire substrate, a SiC substrate or a GaN free-standing substrate, etc. A c-axis 22a of the substrate 22, which is the base of the crystal growth, is inclined so as not to be orthogonal to a surface 22b of the substrate 22. That is, the surface 22b which is not orthogonal to the c-axis 22a of the substrate 22 is exposed, and the crystal growth of the group-III nitride single crystal 24 is performed on the exposed surface 22b. By adjusting an angle θ formed by a normal line n of the exposed surface 22b and the c-axis 22a of the substrate 22 to an appropriate value, a c-axis 24a of the group-III nitride single crystal 24 formed by crystal growth on the exposed surface 22b is also inclined relative to the normal line n, and step-flow growth of the group-III nitride single crystal 24 is promoted. Below, the rotation angle θ from the normal line n of the exposed surface 22b to the c-axis 22a of the substrate 22 is called a miscut angle.
Techniques for promoting crystal growth of a group-III nitride single crystal on a silicon single crystal substrate are taught in Patent Document 1 listed in [0009] or Non-Patent Document 6 listed in [0010]. In these techniques, the growth of mixed crystal, such as AlN or Al(Ga, In)N on the silicon single crystal substrate is promoted, and crystal growth of the group-III nitride single crystal thereon is promoted. Non-Patent Documents 7 and 8 will be described later.